<head>
<title>ЫСлдывдлы</title>
<? include('style.php'); ?>
</head>
<body>
<form action="" method="post">
<input type="text" name="bgcolor" />
<input type="submit" value="Отправить">
</form>
</body>//style.php
<style>
body {
background:#<?= $_POST['bgcolor'];?>;
}
</style>